1. Field of the Invention
The present invention relates to an internal combustion engine which is provided with a decompression mechanism for reducing the compression pressure in the combustion chamber to facilitate the startup of the internal combustion engine when the internal combustion engine is starting to operate.
2. Description of Background Art
An internal combustion engine is known that is provided with a decompression mechanism for reducing the compression pressure when the internal combustion engine starts to operate. See, for example, Japanese Patent Laid-Open No. 2008-19845.
The decompression mechanism reduces a load on the internal combustion engine when it is starting to operate, by opening exhaust valves in timed relation to a compression stroke at the startup of the internal combustion engine. Normally, the decompression mechanism is incorporated in camshaft portions for rotationally actuating the exhaust cams.
In the internal combustion engine disclosed in Japanese Patent Laid-Open No. 2008-19845, an intake cam and an exhaust cam are provided on a camshaft that rotates in ganged relation to a crankshaft, and a plunger that serves as a decompression operator is projectably and retractably mounted on the camshaft at a position adjacent to the exhaust cam. The plunger is projected or retracted depending on the rotation of a decompression shaft supported on the camshaft, and the decompression shaft is angularly moved in response to forces from a return spring and a decompression weight. The return spring normally urges the decompression shaft to turn in a direction to project the plunger, and the decompression weight turns the decompression shaft in a direction to retract the plunger under centrifugal forces depending on the rotational speed of the camshaft. The plunger is projected or retracted at a position adjacent to a base-circle portion of the exhaust cam.
When the internal combustion engine starts to operate, the plunger is projected by the biasing force from the return spring, and pushes an exhaust valve in a valve opening direction through an exhaust rocker arm at the timing of a compression stroke of the internal combustion engine. As a result, the exhaust valve opens an exhaust port at the timing of the compression stroke initiated by the exhaust valve, reducing the compression pressure in the combustion chamber. When the rotational speed of the camshaft reaches a prescribed rotational speed or higher after the internal combustion engine has started to operate, the decompression weight retracts the plunger to cancel the decompression process.
The internal combustion engine disclosed in Japanese Patent Laid-Open No. 2008-19845 is an internal combustion engine of the type wherein two intake valves and two exhaust valves are disposed in one cylinder. On the camshaft, there are disposed a single common exhaust cam for opening and closing the two exhaust valves and two intake cams for opening and closing the two intake valves individually. The exhaust rocker arm has a cam operator (roller abutment) on one end thereof and a bifurcated valve actuator extending on the other end, which is spaced from the one end across a rocker shaft, for pushing the two intake valves. When the internal combustion engine starts to operate, the cam operator on the exhaust rocker arm is pressed by the plunger as the decompression operator, simultaneously pushing the two exhaust valves open to a small degree.
In the internal combustion engine disclosed in Japanese Patent Laid-Open No. 2008-19845, the compression pressure in the combustion chamber can quickly be reduced by opening the two exhaust valves at the time the internal combustion engine starts to operate. With the internal combustion engine, however, since the exhaust rocker arm is of a structure having the single cam operator and the bifurcated valve actuator, it is necessary that the exhaust rocker arm, which is symmetrically shaped, be disposed at an intermediate position between the two exhaust valves. Therefore, the layout of the rocker arm is limited. If the rocker arm is asymmetrically shaped, then the rocker arm needs to be of an increased thickness for reducing twisting and vibration of its own when the internal combustion engine is in normal operation, making it difficult to reduce the weight of a valve actuating mechanism.